1. Field of the Invention
The present invention relates to a laser drive for driving a laser, an optical disc apparatus, and a laser-driving method.
2. Description of the Related Art
A typical optical disc apparatus includes an optical pickup having a semiconductor laser diode as a light source, and the semiconductor laser diode is generally driven by a laser drive.
The radiation power characteristic of the semiconductor laser diode varies depending on individual differences, the ambient temperature, and aging. FIG. 5 is a graph showing the characteristic of radiation power versus forward voltage of a semiconductor laser diode. As shown in the figure, the characteristic shown by the bold line in the graph shifts to those shown by thin lines depending on the ambient temperature, aging, etc., of the semiconductor laser diode. In addition to these factors, individual differences of the semiconductor laser diode also affect the radiation power characteristic. Accordingly, a voltage set for obtaining, for example, the radiation power shown by the dashed line in the figure varies.
FIG. 6 is a diagram showing the structure of a known laser drive that compensates for the characteristic variation. Referring to the figure, the laser drive includes a constant voltage source 60, a laser source 20, such as a semiconductor laser, and a laser drive circuit 70. A constant voltage VLD′ and a current ILD′ are supplied to the laser source 20 and the laser drive circuit 70 from the constant voltage source 60. A voltage Vop′ is applied between the terminals of the laser source 20, and a voltage Vtc′ is applied to the laser drive circuit 70. Here, the following equation is satisfied:VLD′=Vop′+Vtc′  (1)
In order to cause the laser source 20 to emit laser light with a desired radiation power, the laser drive circuit 70 adjusts Vtc′ (Vop′) and ILD′ to control the energy supplied to the laser source 20 (Vop′, ILD′).
However, since Vop′ varies depending on the radiation power, individual differences, the ambient temperature, aging, etc., of the laser source 20, VLD′ is set to a voltage including an allowance to ensure that a sufficient voltage can be supplied even under the worst condition in a predetermined condition range. Therefore, VLD′ is too high under normal conditions, and Vtc′ becomes relatively high as a result. Accordingly, the laser drive circuit 70 consumes excess power as thermal energy, which generates heat that reduces the reliability of the laser source 20 and other peripheral devices.
Japanese Unexamined Patent Application Publication No. 2003-168232 discloses a structure that suppresses an increase in the ambient temperature due to heat generated by a laser drive section by detecting the level of a laser operating voltage and controlling the level of a power source voltage supplied to the laser drive section such that power consumption of the laser drive section maintains constant. Accordingly, variation in the characteristic of current applied to a semiconductor laser versus amount of light emitted therefrom (I-L characteristic) due to the ambient temperature variation is reduced.